1. Field of the Invention
The present invention relates to a method and apparatus for ultrasonic non-destructive inspection. More particularly, the present invention is directed to a method and apparatus for ultrasonic inspection having improved electronic coupling between a transducer and the surface of the object under inspection.
2. Description of the Prior Art
Many machines and vehicles experience wear and part degradation during their useful lives, which gets worse as they age. Ultimately such wear can result in complete failure. Depending upon the machine or vehicle, such failure can be catastrophic. For example, for aircraft, a failure in mid-flight may be deadly.
In order to counter the effects of aging and prolong the effective lives of such machines and vehicles, regimens of regular inspection and maintenance have been put in to practice. While some machines and parts of some vehicles can be inspected and maintained easily, wear and fatigue to some parts and vehicles is difficult to discover, as it may not be visible to the naked eye, and/or be located in an easily accessible location.
Consequently, inspection of such parts and vehicles may be very laborious, expensive and time-consuming. Significant disassembly and reassembly may be required, and the inspection may require considerable test equipment. Previously, regular inspection has often been destructive in the sense that portions of the device or vehicle had to be expended in order to perform a complete inspection. Such destructive examination is extremely expensive and can often be very time consuming.
In an attempt to overcome the drawbacks of such destructive inspection, non-destructive inspection (NDI) has been developed. NDI is typically performed by subjecting the device to be inspected with acoustic waves and then analyzing the reflected waves to determine the state of the device without causing damage to the device. Acoustic NDI is particularly suited for determining the integrity of airplane components. Conventional acoustic NDI equipment emits acoustic waves through a transducer. The reflected acoustic waves are received by the transducer, which produces an electrical signal that is subsequently analyzed to rate the status of the workpiece.
Over time aircraft experience fatigue as a result of normal use of the aircraft. Such fatigue often manifests itself as cracking. In order to prevent failure of the aircraft, which may result in the loss of life, the aircraft are regularly inspected to determine the integrity of the aircraft and to assess the extent of any fatigue experienced by any parts of the aircraft.
An apparatus for acoustically inspecting a workpiece is taught in U.S. Pat. No. 5,469,744 (Patton et al.). The apparatus disclosed in the Patton et al. patent is known as a contact adaptive bubbler. As noted in the Patton et al. patent, ultrasonic NDI can improve the inspection spatial resolution and signal to noise ratio by using a focused acoustic beam. However, to be reasonably effective an ultrasonic transducer needs a good acoustic coupling between the transducer and the workpiece. Water is typically used as a coupling fluid between the workpiece and the transducer, and is sent through a perforated membrane to come in contact with the workpiece.
In order to better control the flow of coupling fluid, the Patton et al. apparatus uses a non-perforated membrane, and two fluid chambers. The non-perforated membrane separates the two chambers. The lower chamber is formed when the apparatus is placed adjacent the workpiece, and can conform somewhat to the shape of the workpiece. The lower chamber is smaller than the upper chamber to provide control of the water flow as compared to an apparatus having a perorated membrane, since apparatus with perforated membranes cannot control the amount of fluid leaking through the membrane.
Conventional bubbler systems including the Patton et al. apparatus utilize a membrane between the transducer and the workpiece to contain the water and reduce air bubble activity. The use of a membrane in such systems creates a drawback, namely the introduction of additional attenuation to the ultrasonic signal.
Furthermore, conventional bubbler systems require a relatively high water flow rate to maintain the water path requirements of automated canning. Such high flow rates are not conducive to efficient couplant removal.
Another drawback, in particular for the inspection of the upper surfaces of aircraft, is that conventional bubblers do not have sufficient couplant recovery and leave significant water residue, which can be a safety hazard.
The deficiencies of the conventional methods are addressed by the present invention that is directed to a method and apparatus for non-destructive inspection. In particular, the method and apparatus of the present invention use an automatic couplant delivery system that delivers and maintains a constant water supply to a single path chamber located at the transducer head. Any couplant run-off is removed and reclaimed by vacuum recovery. A transducer holder block that supports two transducers is used, and mounts to a manifold and brush subassembly.
The manifold and brush subassembly includes a manifold block, a couplant containment/removal block, and gimbal. The manifold and brush subassembly has three sets of brushes that in turn provide three chambers for water or vacuum. An innermost chamber sets the water path, the middle chamber may provide water or regulated vacuum, and the outer chamber provides full vacuum for water removal. The bottoms of the transducers are positioned below the upper surface of the water chamber to allow air bubbles to naturally migrate to the highest point and then evacuate through a vacuum port. Such a configuration eliminates the need for a membrane containment area and then an additional containment chamber. Couplant flow rate and vacuum are selectable and can be adjusted by an operator during initial set up.
An advantage of the method and apparatus of the present invention is that no membrane is needed to contain the couplant in the acoustic transducer apparatus.
Another advantage of the method and apparatus according to the present invention is that additional attenuation is reduced.
Yet another advantage of the method and apparatus according to the present invention is that a majority of any excess couplant is removed.
Still another advantage of the method and apparatus according to the present invention is that the apparatus can be configured for operation above and below a workpiece.
Another advantage of the method and apparatus according to the present invention is that interference from upwardly migrating bubbles is reduced.